I would probably work backwards, ignoring the mashing/boiling device and focusing on the bits you’ll need regardless.
If it was me I would get a fridge to ferment in, which can be second hand, and fit it with a heat pad and an inkbird. If you get lucky on eBay you should be able to get all of these for £100 depending on the fridge.
A fermenter is obviously handy, and a brew bucket should do fine, together with a airlock and grommet. I would get one without a spigot as it’s one less thing to get full of gunge. You can either get one from a cafe/caterer for free (although perhaps now is not the best time), or pick one up online for £10-15.
Bottles & mini kegs you already have, so that’s great.
Cleaner & a sanitiser - £25
Autosiphon - £10
So that’s what - £150?
That leaves £350-£850 to spend on a wort-creating device... how many litres are you planning to brew at once?
My preference would be for an all-in-one eBIAB system, and having used both don’t think the Grainfather is worth the premium over the Robobrew in its current iteration.
If you can hold out for the next version of the G30, that might (might) be amazing, but it may be a long time coming and I think something cheaper like the Robobrew is better value.
I have only used a three vessel system a handful of times, so take this with a pinch of salt, but I found them too big, too much of a pain to set up, and that there was too much cleaning at the end!
Hopefully that’s a start at least. I’m sure others will chime in at some point with more thoughts, including those who have had more experience with 3V than me.